1. Field of the Invention
The invention relates to an exhaust gas sensor.
2. Description of Related Art
Gas sensors that utilize sensor elements have been used conventionally in order to detect specific gases in the atmosphere. Such gas sensors, for instance, can detect the concentration of a specific gas component such as hydrocarbons (HC), oxygen (O2) or the like that are present in the exhaust gas of automobiles. To that end, one such gas sensor is provided in the exhaust passage of an internal combustion engine, for instance an automobile engine. The gas sensor is used to control an exhaust gas control apparatus. Conventional oxygen sensor elements that detect the concentration of oxygen in exhaust gas include, for instance, oxygen concentration-electromotive force sensors that have a ZrO2 solid electrolyte.
As illustrated in FIG. 9, an oxygen sensor element 90, in the form of a bottomed cylinder, is configured through sequential layering of an inner electrode 94a, a solid electrolyte layer 92, an outer electrode 94b, and a diffusion resistance layer 96. A heater 97 is inserted inward of the inner electrode 94a. Exhaust gas reaches the outer electrode 94b through micro-holes in the diffusion resistance layer 96, and a sensor output is obtained between the outer electrode 94b and the inner electrode 94a. The diffusion resistance layer 96 is formed of a porous ceramic coating, with a view to imparting a function of limiting the flow rate of exhaust gas that reaches the outer electrode 94b and of protecting the outer electrode 94b. 
The exhaust gas contains noxious substances that include components present in oil, for instance P, Ca or Zn, as well as gasoline additive components such as K or Na. Therefore, in cases where such a sensor element is used in an exhaust gas sensor for automobiles, a problem may arise in that the sensor element may become contaminated by these noxious substances. To address this problem, it has been proposed to cover the periphery of the sensor element with a porous protective layer (trap layer) 98 (FIG. 9). In the above technology, noxious substances in the exhaust gas become adsorbed onto the protective layer 98. As a result, this allows suppressing intrusion of the noxious substances into the sensor element. Japanese Patent Application Publication No. 2011-252894 (JP 2011-252894 A) is an instance of related art pertaining to such gas sensors.